Removed pins django app, and moved code to the core. Moved user related
code out of core to the users app.

As pointed in issue #75 we should get away with just checking if the pin
submitter is the currently logged in user. Assuming that we can implement
authorization for updating and deleting pins rather easily by subclassing
DjangoAuthorization so it passes the object to the Authorization backend.

This is going to be used by a "similar images" feature #74

Rewritten register view using CBV approach, and added a authenticate/login
call after form has been validated and user created. Fixes #65

It didn't make sense to call it upload-pin as it is being used only for
creating the Image object.

Fixes #81

Fixes #68

Update JavaScript so it orders pins, Fixes #69

Rewritten API to handle creating pins for both urls
and previously-uploaded images. Added some tests for it.

Generate thumbnail and standard image on request, and use
http://github.com/mirumee/django-images for generating them. Also,
remove the CreatePin page as pin creation is going to be done
in JavaScript. Create UploadImage view for uploading images from
computer.

REST API should be used to delete pins anyway, so remove the old code
that is not being used.

I have a gut feeling that we could refactor some code out of this method, but for now I'll just clean it up a bit.

There has been some refactoring going on in the pinry.pins.models module.
The upload_to code has been refactored into its own function, images
have been moved to their own models - otherwise the number of fields
in the Pin model would skyrocket. Also ModelManagers have been written
to move image fetching and generating outside of models.